This invention relates to an improved tool for short to intermediate production run use.
Complex-shaped plastic articles are typically formed by either injection molding or compression molding techniques. Typically, the tool or mold utilized in the molding operation is produced from a metal such as steel. The utilization of a tool made from steel is generally appropriate when the tooling will be used to produce in excess of 50,000 components from the tool. Because of the high cost of producing a tool made from steel, a steel tool is generally only selected when large numbers of components will be produced from the tool as the cost can be more easily amortized over a large number of components. However, when the desired quantities of a plastic article or component only range from hundreds to tens of thousands, the cost of producing a machined steel tool can often be prohibitive.
Since aluminum can be machined more rapidly than steel, aluminum has been a somewhat less expensive alternative to steel. Additionally, aluminum tooling is not generally suitable for filled plastics and/or intermediate to high volume use due to its poor abrasion resistance.
In addition to the expense associated with steel and aluminum tooling, it is necessary in the plastics industry to have the capability to rapidly prototype plastic components by utilizing techniques which are as close as possible to those techniques which will be used in the actual production process. For example, in the automobile industry, an automobile manufacturer may wish to field a small test fleet of vehicles. It is often the goal of these test fleets to closely mimic the form, fit, and the function of the production vehicle. For this use, steel or aluminum tooling could be too expensive.
Cast tooling provides the ability for short run, rapidly fabricated tooling. Cast tooling is commonly formed of castable materials such as epoxy, silicone or urethane resins, with or without ceramic or metal fillers. In the process of fabricating a cast tooling, a pattern is made from a suitable material, such as plastic, wood, steel, and/or aluminum, and the pattern is placed in a cavity. A castable material such as epoxy is then poured around the pattern. Then the epoxy cures and the pattern is removed. The resultant tool or mold is suitable for producing parts in quantities of generally less than 100 pieces.
While cast tooling is much less expensive than traditional steel tooling it is subject to several drawbacks. First, the cast tool has a reduced strength and elastic modulus compared to a metal tool and distorts under the pressure exerted under typical injection molding and compression molding applications. Second, the cast tooling has a lower thermal conductivity than a metal tool which can result in longer mold cycle times and can contribute to difficulty in solidifying plastics in articles having thicker cross-sections. Third, the cast tool is mechanically weak as compared with metallic tools. This problem manifests itself in the breakage of thin cross-section mold areas during use. Finally, the cast tools can typically only be used in a temperature range of approximately 300 to 350.degree. F. (150 to 180.degree. C.) which can limit the types of plastics or filled plastics that can be utilized in combination with the cast tool for injection molding.
In an attempt to improve the thermal conductivity of polymer based cast tools, such as the epoxy castings, additives such as aluminum or silicon carbide powder or fillers have been added to the castable material. These types of additives are dispersed within the castable material in a discontinuous manner. These fillers (e.g., aluminum) also provide a mechanism to channel the heat given off (exotherm) produced during resin curing and impart additional abrasion resistance to the cast tool (e.g., silicon carbide). However, in general, it was found that the addition of these powders or fillers to cast systems reduce the mechanical strength of the cast tool relative to its unfilled resin by producing defects in the cured structure. This use of discontinuous fillers results in marginal cast tooling; limiting the total number of parts that can be made.
Accordingly, it would be desirable and advantageous to have a method for rapidly forming tooling for plastic molding which is suitable for use in short to intermediate length production runs, which is less expensive than metallic tooling, such as steel, and which provides the ability to prototype parts using a "near production" process so that all aspects of both the prototype part and the production thereof can be analyzed and observed. This includes the ability to affect heat transfer rates during molding that mimic the heat transfer rates of production steel tooling.